1. Field of the Invention
The invention relates to a process for the preheating and multi-stage degassing of make-up water by means of steam in a power generation installation. It also relates to equipment for carrying out the process.
In modern power station installations, the consumption of make-up water is very high. Thus, even conventional condensation units normally require, as make-up water, 1 to 3% of the total circulating mass flow of the steam/condensate circulation. In combination installations, however, where the emission of nitrogen oxides is reduced by means of steam injection, this consumption of make-up water can rise up to 20 to 30%. Owing to the current requirements, a degassing range of the O.sub.2 in make-up water from 10,000 ppb (parts per billion)--i.e. the state of saturation of the water with atmospheric air at room temperature--down to 7 ppb is then demanded, with simultaneous heating by 20K and more.
2. Discussion of Background
The simple, direct spraying, known per se and conventional in the case of small water rates, of makeup water into the condenser would cause flooding of the tube bundles, because of the large quantities mentioned. This would entail severe impairment of the functioning of the condenser and is therefore not permitted, either by the condenser supplier or by the condenser operator. Moreover, the desired preheating and degassing cannot be achieved in this way.
The use of the generally known types of equipment for preheating and degassing of the make-up water normally has the result that the steam used for the preheating is at a relatively high energy level. Accordingly, a means is used which could readily still perform work in the turbo group.
In connection with the degassing of liquids, exchange towers or columns are also known in which the vapor and the liquid of a mixture flow in counter-current, in cross/counter-current or in co-current relative to one another (LUEGER, volume 16, Lexikon der Verfahrenstechnik [Dictionary of Process Engineering], fourth edition, Deutsche Verlags-Anstalt Stuttgart, page 51). In these columns, the liquid and the vapor of a mixture which is to be separated flow in counter-current or in co-current relative to one another in such a way that both phases are in the most intimate contact possible for mass transfer and heat exchange. The use of a degasser column in counter-current arrangement is correct in principle for achieving a maximum separation effect at a defined column height. In the case of a counter-current arrangement, however, greater liquid flows having greater subcooling lead to the problem that a very large part of the vapor must overcome the lower part of the column (flooding of the packing because of higher vapor loading), without significant action on the mass-kinetic separation effect, which occurs in the lower part of the column, since the preheating and the saturation process of the liquid take place in the upper part. The consequence of such a design solution is, however, that the diameter of the column must be enormously increased in order to prevent flooding of the packing, with considerable additional costs for the equipment.
Finally, it is also known, at least in connection with the corrosion problem in the feed water train of steam turbine installations, to carry out a post-degassing of the condensate by blowing steam into the hot well below the water level. A sufficiently large coverage with condensate and a sufficiently fine steam distribution in the condensate are hen necessary for blowing in steam.